1. Field of the Invention
The present invention relates to an exhaust gas purification catalyst for the purification of exhaust gas emitted from internal combustion engines of automobiles and the like and, specifically, it relates to a ceramic carrier which is ideal as a carrier for an exhaust gas purification catalyst in a lean burn engine or diesel engine, and to a ceramic catalyst body comprising it.
2. Description of the Related Art
xe2x80x9cThree-way catalystsxe2x80x9d have been widely used in the past for simultaneous purification of CO, HC and NOx emitted from automobiles. Recent years have brought a further demand for cleaner exhaust gas and reduced CO2 emissions in order to protect the natural environment, and various xe2x80x9clean burnxe2x80x9d systems have come into use to allow reduction in exhaust gas volumes through improved fuel efficiency. However, since conventional three-way catalysts have reduced NOx purification performance at the lean end (region of oxygen excess), the inherent performance cannot be exhibited; NOx storage reduction catalysts have therefore been developed to compensate for this problem. In addition to the precious metals, such as Pt and Rh, used for common three-way catalysts, NOx storage materials which store NOx in lean atmosphere conditions and release and purify stored NOx under stoichiometric (theoretical air/fuel ratio) to rich atmosphere conditions are added as cocatalysts, and the NOx storage materials are also used with highly basic alkali metals such as Na, K and Cs or alkaline earth metals such as Mg, Sr and Ba.
NOx storage reduction catalysts are described, for example, in Japanese Unexamined Patent Publication HEI No. 6-31139, which discloses a catalyst prepared by coating a porous body of xcex3-alumina, etc. onto a honeycomb carrier composed of a ceramic such as cordierite, a low thermal expansion material which has excellent heat resistance, and loading an alkali metal oxide and Pt, thereby allowing reduction in NOx emissions under lean conditions. However, since the HC purification performance is lowered if the basicity of the alkali metal used as the NOx storage material is too strong, the NOx storage material is selected to match the desired performance.
On the other hand, exhaust gas temperatures have also been increasing in recent years, making it important to improve the high temperature durability of exhaust gas purification catalysts. Incidentally, catalysts with alkali metals loaded as NOx storage materials on cordierite carriers have been associated with the problem of reduced NOx storage capacity and cordierite carrier impairment under higher exhaust gas temperatures. This is attributed to the fact that the alkali metal easily penetrates into the porous coating layer of xcex3-alumina and reacts with the Si in the cordierite; as a measure against this, Japanese Unexamined Patent Publication HEI No. 10-165817 proposes using a carrier made of a low thermal expansion material containing no Si, instead of a cordierite carrier.
However, of the xcex1-alumina, zirconia, titania, titanium phosphate, aluminum titanate, stainless steel and Fexe2x80x94Alxe2x80x94Cr alloy mentioned as examples in Japanese Unexamined Patent Publication HEI No. 10-165817, only the very highly dense (heavy) aluminum titanate exhibits a sufficiently low thermal expansion coefficient for practical use. Aluminum titanate, however, is poorly suited given the trend toward lighter weight vehicles and its high cost increases the cost of the metal carrier. Other ceramic materials have high thermal expansion coefficients, and are also impractical from the standpoint of impact resistance. Thus, it is the current situation that no low-cost carrier material with a low thermal expansion coefficient exists as a substitute for cordierite.
Japanese Unexamined Patent Publication HEI No. 10-137590 discloses an exhaust gas purification filter wherein an alkali metal and an alkaline earth metal are carried on a coating layer comprising at least one from among silica, zirconia, titania and silica-alumina provided on a ceramic carrier, and it is stated that the coating layer inhibits diffusion of the catalyst components into the filter. However, research by the present inventors has shown that these coating layer materials produce compounds by reaction with the alkali metals and alkaline earth metals under conditions of approximately 800xc2x0 C., which is the temperature at which exhaust gas purification catalysts are generally used. That is, under high temperature conditions of 800xc2x0 C. and above, the alkali metals and alkaline earth metals react with the coating layer, while the excess alkali metals and alkaline earth metals diffuse to the interior reaching the filter surface, and can also react therewith. Thus, under the current situation in which the maximum exhaust gas temperatures can reach up to around 1000xc2x0 C., it has been difficult to inhibit diffusion of alkali metals and alkaline earth metals into coating layers made of such materials.
It is an object of the present invention to realize ceramic carriers and ceramic catalyst bodies which have low cost and excellent high temperature durability, while not exhibiting reduced catalytic performance due to reaction with the alkali metals and alkaline earth metals carried as cocatalysts and used as NOx storage materials, and which can maintain the necessary catalytic performance over long periods when used as exhaust gas purification catalysts in lean burn engines.
According to a first aspect of the invention there is provided a ceramic carrier prepared by forming a diffusion-inhibiting layer which inhibits diffusion of the carried catalyst components on the surface of a ceramic honeycomb structure, wherein the diffusion-inhibiting layer is composed of a ceramic material which substantially does not react with the catalyst components under the temperature conditions of use and which has a melting point that is higher than the maximum temperature of use.
Since the diffusion-inhibiting layer substantially does not react with the catalyst components at the use temperature of the catalyst, the catalyst components do not diffuse into the diffusion-inhibiting layer. Thus, it is possible to prevent the diffused-catalyst components from reaching the surface of the ceramic honeycomb structure and reacting with it. Also, since the diffusion-inhibiting layer has a melting point higher than the maximum use temperature of the catalyst, there is no loss of diffusion-inhibiting effect by melting. Consequently, the ceramic honeycomb structure can be constructed of inexpensive and high temperature durable cordierite and can maintain its catalyst performance over long periods, so that it is ideal as an exhaust gas purification catalyst for lean burn engines.
According to a second aspect, the ceramic carrier is one with a thermal expansion coefficient of no greater than 1.5xc3x9710xe2x88x926/xc2x0 C. in the direction of flow. This improves the thermal shock resistance and reduces the risk of thermal shock damage even when used as an exhaust gas purification catalyst through which high temperature exhaust gas flows.
According to a third aspect, the melting point of the ceramic material composing the diffusion-inhibiting layer is 1000xc2x0 C. or higher. Since the maximum temperature never exceeds 1000xc2x0 C. during use as an exhaust gas purification catalyst, a ceramic material with a melting point of 1000xc2x0 C. or higher will not exhibit a reduced function due to melting of the diffusion-inhibiting layer.
According to a fourth aspect, the ceramic honeycomb structure is a material with reactivity for the catalyst components, for example, a ceramic material containing Si, according to a fifth aspect. Ceramic materials containing Si readily react with catalyst components such as NOx occluders, and in such cases, providing the diffusion-inhibiting layer can prevent deterioration by the reaction. Specifically, by using inexpensive cordierite with a low thermal expansion coefficient as the ceramic honeycomb structure as according to a sixth aspect, a considerable effect is achieved in terms of cost reduction and improved thermal shock resistance.
According to a seventh aspect, the catalyst components include at least one selected from among alkali metals and alkaline earth metals. These metals are used as NOx storage materials in exhaust gas purification catalysts and, specifically, according to an eighth aspect, potassium is suitable as a catalyst component due to its high NOx storage capacity. However, there is a concern regarding lower performance due to diffusion into the carrier, and using a ceramic carrier provided with a diffusion-inhibiting layer according to the invention can effectively prevent the diffusion.
According to a ninth aspect, the porosity of the diffusion-inhibiting layer is no greater than 50%. By forming a diffusion-inhibiting layer with a porosity of no greater than 50%, it is possible to suppress diffusion to prevent penetration of the catalyst components into the ceramic honeycomb structure under normal conditions of use.
According to a tenth aspect, the ceramic material composing the diffusion-inhibiting layer is a metal oxide containing at least one selected from among non xcex1-alumina, Ni, Cu, Zn, Y and lanthanoid elements. These ceramic materials have melting points of above 1000xc2x0 C. and do not react with the catalyst components according to the seventh aspect, so that their diffusion can be reliably inhibited to maintain catalytic performance.
According to an eleventh aspect, the thickness of the diffusion-inhibiting layer is no larger than the mean pore size of the ceramic honeycomb structure. If it is larger than the mean pore size, the pores of the honeycomb structure become clogged and adhesion to the coating layer of xcex3-alumina, etc. formed on the diffusion-inhibiting layer easily deteriorates, but this can be prevented by keeping the thickness lower than the mean pore size.
According to a twelfth aspect, the diffusion-inhibiting layer is formed by a dip method, a PVD method or a CVD method. All of these methods allow satisfactory formation of the diffusion-inhibiting layer on the surface of the ceramic honeycomb structure, to inhibit diffusion of the catalyst components.
According to a thirteenth aspect, the diffusion-inhibiting layer is formed by a dip method in which a series of steps including immersion in the dip solution, drying and firing is repeated a plurality of times. The fine cracks generated during the process of immersion in the dip solution, drying and firing are reduced by repeating the immersion in the dip solution, drying and firing, to allow formation of a higher quality diffusion-inhibiting layer with fewer cracks in the surface, and to thereby provide a greater effect of inhibiting diffusion of the catalyst components into the honeycomb structure under conditions of use.
According to a fourteenth aspect, the diffusion-inhibiting layer is formed by a dip method in which the dip solution used is a slurry prepared by uniformly dispersing particles of the ceramic material into a water-soluble or water-insoluble solvent. By immersion in a dip solution in which the ceramic particles are dispersed uniformly and preferably in the form of primary particles, it is possible to minimize the number of uncoated sections resulting from aggregation between the ceramic particles during drying and firing, and to minimize creation of cracks due to poor film thickness uniformity, for a greater effect of inhibiting diffusion of the catalyst components into the honeycomb structure under conditions of use.
According to a fifteenth aspect, the diffusion-inhibiting layer is formed by a dip method in which the dip solution used is a solution wherein a starting material for the ceramic material is uniformly present in ion form in a water-soluble or water-insoluble solvent. By immersion in a dispersion solution in which the metal elements of the ceramic material are uniformly present in ion form, followed by treatment in a prescribed gas atmosphere, it is possible to allow the dip solution to penetrate to the fine sections of the ceramic honeycomb structure, to form a diffusion-inhibiting layer with no uncoated sections for a greater catalyst component diffusion-inhibiting effect.
According to a sixteenth aspect, at least one intermediate layer is formed between the ceramic honeycomb structure and the diffusion-inhibiting layer, the intermediate layer comprising a ceramic material with a different thermal expansion coefficient and with a higher melting point than the maximum temperature of use.
The intermediate layer has a different thermal expansion coefficient than the ceramic honeycomb structure during the course of the temperature history including the diffusion-inhibiting layer formation temperature conditions and the temperature conditions in which the catalyst is used, so that stress and cracking in the diffusion-inhibiting layer is reduced, and the effect of the diffusion-inhibiting layer which inhibits diffusion of the catalyst components into the ceramic honeycomb structure can be adequately exhibited. Since the intermediate layer also has a melting point which is higher than the maximum use temperature of the catalyst, there is no reduction in the diffusion-inhibiting effect due to melting.
According to a seventeenth aspect, the thicknesses of the diffusion-inhibiting layer and the intermediate layer are no greater than the value of the mean pore size of the ceramic honeycomb structure. By limiting the thicknesses to a smaller value than the mean pore size, it is possible to prevent clogging of the honeycomb structure pores by formation of the diffusion-inhibiting layer and intermediate layer, and the consequent loss of adhesion with the coating layer of xcex3-alumina, etc. to the diffusion-inhibiting layer.
According to an eighteenth aspect, the intermediate layer is formed by a dip method, a PVD method or a CVD method. All of these methods allow satisfactory formation of the intermediate layer on the surface of the ceramic honeycomb structure, to increase the catalyst component diffusion-inhibiting effect of the diffusion-inhibiting layer.
According to a nineteenth aspect, the intermediate layer is formed by a dip method in which a series of steps including immersion in the dip solution, drying and firing is repeated a plurality of times. This gives a higher quality intermediate layer with fewer cracks in the surface, as according to the thirteenth aspect, and allows satisfactory formation of the diffusion-inhibiting layer thereover to effectively inhibit diffusion of the catalyst components.
According to a twentieth aspect, the intermediate layer is formed by a dip method in which the dip solution used is a slurry prepared by uniformly dispersing particles of the ceramic material into a water-soluble or water-insoluble solvent. This gives a higher quality intermediate layer with no uncoated sections or cracks, as with to the fourteenth aspect, and allows satisfactory formation of the diffusion-inhibiting layer to effectively inhibit diffusion of the catalyst components.
According to a twenty-first aspect, the intermediate layer is formed by a dip method in which the dip solution used is a solution wherein a starting material for the ceramic material is uniformly present in ion form in a water-soluble or water-insoluble solvent. This gives a higher quality intermediate layer with no uncoated sections or cracks, as with the fifteenth aspect, and allows satisfactory formation of the diffusion-inhibiting layer thereover to effectively inhibit diffusion of the catalyst components.
According to a twenty-second aspect, the mean primary particle size of the ceramic particles composing the diffusion-inhibiting layer and/or the intermediate layer is no greater than the mean pore size of the ceramic honeycomb structure. Preferably, as according to a twenty-third aspect, the mean primary particle size of the ceramic particles is limited to no greater than {fraction (1/10)} of the mean pore size of the ceramic honeycomb structure, so that the diffusion-inhibiting layer or intermediate layer can be evenly formed on the pore surfaces of the ceramic honeycomb structure, for a greater effect of inhibiting diffusion of the catalyst components in the honeycomb structure under conditions of use.
According to a twenty-fourth aspect, the ceramic material composing the intermediate layer may be a ceramic material with a different crystallinity, anisotropy, constituent components or compound composition than the ceramic materials composing the ceramic honeycomb structure and the diffusion-inhibiting layer, or else a mixture or composite compound of the ceramic materials composing the ceramic honeycomb structure and the diffusion-inhibiting layer. In either case, it is possible to satisfactorily form the diffusion-inhibiting layer thereover to inhibit diffusion of the catalyst components into the honeycomb structure.
A twenty-fifth aspect relates to a ceramic catalyst body, wherein a catalyst component-carrying layer containing the catalyst components is formed on the surface of a ceramic carrier according to any one of the first to twenty-fourth aspects, and it is obtained, for example, by forming a coating layer of xcex3-alumina, etc. on a ceramic carrier of the invention and loading it with a catalyst. Preferably, as according to a twenty-sixth aspect, the catalyst components carried on the catalyst component-carrying layer include at least potassium, and the ceramic carrier comprises a ceramic honeycomb structure and a diffusion-inhibiting layer formed on its surface to inhibit diffusion of potassium. The diffusion-inhibiting layer is composed of a ceramic material which has a higher melting point than the maximum temperature of use and which substantially does not react with potassium under the temperature conditions of use, so that it is possible to inhibit diffusion of potassium into the ceramic carrier and thus achieve both cost reduction and improved catalytic performance.